A cylinder liner of a water-cooled internal combustion engine comes into contact with cooling water at the outer peripheral surface thereof and this leads to cavitation erosion in the areas of the outer peripheral surface coming into contact with the cooling water. The causes of cavitation erosion are chemical corrosion caused by the cooling water and mechanical corrosion caused by vibration of the cylinder liner. It is generally believed that the latter mechanical corrosion is mainly responsible for the cavitation erosion. High-speed vibration of the cylinder liner produces local pressure variations in the cooling water and the local pressure variations cause local formation and disappearance of bubbles. The formation and disappearance of bubbles provides repeated shocks to the cylinder liner which causes the mechanical corrosion. Thus the cavitation erosion is maximum in those directions where the vibration of the internal combustion engine is vigorous, i.e., the so-called thrust and counter thrust directions perpendicular to a crankshaft.
Various proposals have heretofore been made to prevent such cavitation erosion and they can be divided broadly into:
(1) a method of treating the surface of a cylinder liner, and PA1 (2) a method of strengthening the structure of a cylinder liner and a cylinder block. PA1 (1) a remelted white cast iron structure in a part or whole of areas of the outer peripheral surface of the cylinder liner which come into contact with cooling water; and PA1 (2) a thermally affected layer between the white cast iron structure and the parent material.
The methods which are classified into the latter structure-stengthening method (2) include a method in which a post or a fin is provided to prevent the vibration of the cylinder liner in the thrust direction and a method in which a cylinder block or cylinder liner is molded into a corrugated form to disperse the vibration.
Usually, however, the former surface-treating method (1) has been employed. Examples of the surface treating methods include a method in which a rigid chromium layer is plated onto the outer peripheral surface of the cylinder liner, a method in which a sprayed ceramic layer is formed on the cylinder liner, a method in which a steel plate is attached to the outer peripheral surface of the cylinder liner, and a method wherein while casting a cylinder liner a chilled structure is formed in the outer peripheral surface of the cylinder liner by the use of chillers.
With cylinder liners subjected to a cavitation-preventing treatment in accordance with the structure-strengthening method (2), the effect of preventing the cavitation varies depending on the state in which the engine is operated and therefore cavitation may still occur if the engine is not operated properly.
On the other hand, when the surface-treating method (1) is employed, the effect varies depending on the hardness and structural strength of a layer formed in or provided on the outer peripheral surface of the cylinder liner. It has been confirmed experimentally that cylinder liners with a layer having a high hardness and containing no defects in the surface exhibit a high resistance to the impact due to the formation and collapse of bubbles on the outer peripheral surface thereof. For example, a rigid chromium plated surface shows much higher resistance than cast iron in which graphite grains are dispersed (these graphite grains are regarded as defects).
A cylinder liner with a rigid chromium layer which is formed by plating or a ceramic layer which is formed by spraying suffers from various problems which are not desirable from a standpoint of commercial production. In particular, the time required for the production of such layers is long and the starting materials used in these surface treatment methods are expensive.
With a cylinder liner with a chilled structure formed in the outer peripheral surface thereof (as disclosed in Japanese Utility Model Publication No. 25530/1979), the chilled structure (a white cast iron layer) has a high hardness and contains no free graphite. Therefore, the cylinder liner has a high resistance to cavitation. Chilling using chillers, etc., results in the formation of a two layer structure composed of a chilled structure and a parent material. This gives rise to the problems described hereinbelow.
Although a hard layer (a chromium layer or chilled structure) having a thickness of 0.3 mm or less has static conditions independent of the parent material, its dynamic conditions, e.g., fatigue, is influenced by the parent material. The influence of such dynamic conditions on cavitation is not small. Therefore, the hard layer provided on the parent material is required to have a certain minimum thickness. Since the chilled structure has a hardness lower than that of the rigid chromium layer formed by plating and in addition is easily influenced by dynamic conditions, it is necessary to increase the thickness of the chilled structure to a very high level. Moreover, since the chilled structure is formed by forced cooling from the outer peripheral surface, uneven cooling readily occurs. Furthermore, the chilled structure is greatly influenced by the stream of a cast melt. It is therefore very difficult to provide a stable chilled structure having a predetermined thickness. In particular, it is commercially impossible to form a chilled structure having a thin and uniform thickness. For these reasons the thickness of the chilled structure (including the unevenness) is inevitably increased to at least 2 mm. Therefore, when a relatively thin cylinder liner is used, the formation of such a thick chilled structure influences the inner peripheral surface of the cylinder liner and changes the structure and hardness of the inner peripheral surface. Moreover, when chilling reaches near the inner peripheral surface, working becomes difficult.